As a sensor detecting flammable gas such as hydrogen gas and methane gas, a catalytic combustion type gas sensor has conventionally been known. As shown in FIG. 13, for instance, the catalytic combustion type gas sensor uses a sensing element 20 in which a heater coil 23 is buried in a heat conducting layer (catalyst carrier) 22 whose surface is coated with or carries a combustion catalyst layer 21 causing combustion of a detection target gas brought into contact therewith, and the heater coil 23 of the sensing element 20 is heated to a predetermined temperature in advance by supplying electricity therethrough, and when flammable gas comes into contact with the combustion catalyst layer 21 to burn, a change in resistance value of the heater coil 23 occurs due to a temperature increase caused by the combustion and is detected as voltage, whereby the presence of the flammable gas is detected (for example, see a patent document 1).
Further, the same patent document 1 describes a gas sensor as shown in FIG. 14 in whose sensor main body 25, a compensating element 26 (resistance value RC) is serially connected to the sensing element 20 (resistance value RD) in order to cancel the influence by a change in ambient temperature, and this series circuit is connected in parallel to a series circuit composed of two serial-connected fixed resistance elements 27 (resistance value R1) and 28 (resistance value R2), thereby forming a Wheatstone bridge circuit. Direct-current voltage is applied by a power source 29 between both ends of the parallel circuit, and output voltage Vout between a connection point a of the sensing element 20 and the compensating element 26 and a connection point b of the two resistance elements 27, 28 is detected. As the compensating element 26 in this case, used is a element in which a heater coil having the same electric characteristic as that of the sensing element 20 is buried in a heat conducting layer coated with a compensating material layer instead of the combustion catalyst layer.
The aforesaid output voltage Vout of the Wheatstone bridge circuit is dependent on a balance of the electrify resistances (RD and RC) of the sensing element 20 and the compensating element 26. In a clean atmosphere, the resistance value RD and the resistance value RC are determined in a balanced state between heat generation amounts of the heater coils in the sensing element 20 and the compensating element 26 and amounts of heat dissipated into the atmosphere, and the output voltage Vout shows a zero-point value. When the detection target gas comes into contact with the sensing element 20, the temperature of the sensing element 20 increases due to catalytic combustion, and thus only the resistance value RD increases, resulting in an increase in the output voltage Vout, and the target gas is detected based on an amount of this increase.
By the way, in recent years, the practical application of a fuel cell vehicle (hereinafter, abbreviated as “FCV”) which does not use fossil fuel such as gasoline and thus does not involve a possibility of causing air pollution due to exhaust gas has been started, and it is stipulated as compulsory that the FCV is equipped with a gas sensor for high-sensitivity detection of hydrogen leakage.
However, automobile components go through severe tests, for example, dew condensation is made to occur in the components and dew is frozen, or the components are repeatedly ON-OFF operated while exposed to 90° vapor. In all of these tests, moisture is given to the automobile components to check their durability, and components mounted on the FCV are also required to have the same durability.
The catalytic combustion type gas sensor is considered as one of sensors having an operation principle with high total adaptability as a hydrogen sensor for FCV, but cannot be said to have high waterproof performance because of its structure.
The reason for this will be briefly described. A sensing element and a compensating element in a conventional catalytic combustion type gas sensor has a cross-sectional structure as shown in FIG. 13, and the sensing element 20 has a sponge-like cross-sectional structure allowing the permeation of hydrogen so that hydrogen combustion activity can be obtained also in a thickness direction of the combustion catalyst layer 21, and thus steam, minute water droplets, and the like can enter the combustion catalyst layer 21.
Further, materials forming the combustion catalyst layer 21 are “tin oxide+iron oxide+platinum fine powder+palladium fine powder+others”, and the combustion catalyst layer 21 exhibits a high characteristic with respect to a retaining capability of water droplets and the like owing to a hydrophilic property of these materials.
The compensating material layer of the compensating element does not have a sponge-like cross sectional structure, but it has a hydrophilic property as in the sensing element because its constituent materials are “tin oxide+copper oxide+others”.
On the other hand, the heat conducting layers positioned on inner sides of the combustion catalyst layer and the compensating material layer of the sensing element and the compensating element have a dense structure and their constituent materials are “alumina+titania (titanium oxide)+boron nitride+bismuth oxide glass+others” and so on, and the heat conducting layers exhibit a hydrophobic property owing to these factors.
Therefore, these constituent layers peel off each other in a high-humidity environment due to a difference in hydrous property therebetween. In particular, when dew condensation occurs in the sensing element and the compensating element and the dew is frozen, the combustion catalyst layer and the compensating material layer drop from surfaces of the heat conducting layers. Consequently, the electrify resistance values of the sensing element and the compensating element change from initial values and accordingly, a zero point output value of the output voltage of the Wheatstone bridge circuit greatly fluctuates, resulting in error detection.
Therefore, some FCV makers take measures which involve high cost factors, such as setting a special environment for protecting a hydrogen sensor from water, and improvement in waterproof performance of the hydrogen sensor itself is required in order to promote wider use of the FCV.
As a technique for enhancing durability of such a conventional gas sensor, it has been proposed in, for example, a patent document 2, that a heater heating a detection target gas is provided adjacent to and on an upstream side of a gas sensor installation position in a channel through which the detection target gas flows, thereby preventing dew condensation in the gas sensor.
Patent document 1: JP H 3-162658A
Patent document 2: JP 2004-69436A